In conventional disc brake assemblies for wheeled motor vehicles, a vertically disposed disc is rigidly attached to the wheel to be braked and a caliper is disposed about that disc so that braking surfaces, held by the caliper, are moveable towards opposite vertical sides of the disc. As a part of the caliper, at least one fluid operated piston, e.g. hydraulic or air, exerts pressure on the braking surfaces to move those braking surfaces into engagement with the disc. The fluid operated piston is provided with pressurized fluid via convenient mechanical devices, such as foot pedals and the like. The caliper is secured to an iron anchor frame, which is rigidly attached to the support for the wheel to be braked. A keyway is provided, which is defined by sliding surfaces on the caliper, a caliper recess, and an adjacent surface of the anchor frame. A key is disposed in that keyway. The key is fixedly attached to the anchor frame and the key has surfaces for providing lateral sliding movement between the caliper sliding surfaces and the key sliding surfaces and for providing a positive lock against substantially vertical displacement of the caliper from the key. A caliper support spring is disposed between the key and a surface of the caliper recess for resiliently and slideably supporting the caliper sliding surfaces on the key sliding surfaces. That support spring is of a configuration such that when disposed within the keyway a vertical force of about 12-14 g's on the caliper is required for the caliper to fully compress the spring, which provides an effective filler or spacer between the caliper sliding surfaces and the key sliding surfaces.
Thus, when fluid pressure is applied to the piston in the caliper, the braking surfaces are engaged with the disc of the wheel to be braked and the caliper slides laterally on the key. By such sliding, the braking surface, opposite to the braking surface engaged by the piston, is moved into contact with the opposite surface of the disc at the same time the braking surface operated by the piston is moved into braking engagement with the disc.
In conventional disc brakes, no positive means are provided for moving the braking surfaces away from the disc, once the fluid pressure on the piston has been released. The design of these conventional brakes intends that normal road vibrations will be transmitted to the disc in the form of lateral amplitudes and such vibrations will cause braking surfaces are disengaged from the disc.
Also, in most conventional disc brakes, especially for automobiles, all of the anchor frame, caliper, key, and key support springs are made of iron or at least ferrous metals. During use of the disc brakes, water and atmospheric corrosive materials can collect on the above noted sliding surfaces of the caliper, the key, and the caliper support spring. Over a period of time the ferrous metals of the caliper, key, and support spring begin to rust. An accumulation of galling or binding rust between the key sliding surfaces, the caliper sliding surfaces or the support spring continues to increase and the frictional resistance to the sliding of the caliper on the key, i.e. the "effective sliding friction", correspondingly increases. This increase in resistance to the caliper sliding on the key, is not of substantial concern in applying the brakes, since, typically, the piston in the caliper will exert two or more tons of pressure on the caliper. Thus, that exceedingly high pressure is quite capable of sliding the caliper into braking engagement, even though substantial galling and binding rust has accumulated. However, since no positive means are provided for disengaging the braking surfaces once the fluid pressure on the piston has been released, the frictional resistance can become so great that normal road vibrations are insufficient to disengage the braking surfaces. The pressure of this retained engagement will be considerably less than the pressure exerted during braking, but nevertheless that retained pressure on the braking surfaces is quite substantial.
This retained pressure on the braking surfaces results in two most unwanted occurrences. First of all, the friction between the engaged braking surfaces and the disc generates substantial heat which can damage both the disc and the braking surfaces and can result in the brake becoming substantially unoperable or at least the efficiency thereof substantially reduced. Secondly, the friction caused by the retained engagement of the braking surfaces substantially increases the power required to drive the motor vehicle, with a considerable reduction in fuel efficiency.
The foregoing problem is accentuated by the caliper support spring. Since the caliper in conventional disc brakes may be relatively heavy, e.g. from 4 to 15 pounds, a considerable vertical force, measured in g's, is exerted on the caliper with vertical movement of the wheel. For example, if the wheel encounters a pothole, with rapid vertical up and down movement, the force exerted on the caliper may be in the range of 5 to 8 g's, depending upon severity of the pothole. In extreme vertical movement of the wheel, such as the wheel running over ties of a railroad track, the vertical force exerted on the caliper can reach 9 g's or more, and in some cases as high as 10 to 12 g's.
Thus, the typical caliper support spring is designed such that the vertical force on the caliper required for the caliper to fully compress the support spring is at least 12 g's, and more usually 12 to 14 g's. With such a support spring, the brake is capable of passing over severe irregularities in the road without causing a clanking sound which would otherwise result when the caliper under high g loads forceably strikes the key.
However, the force of the support spring, being a frictional normal force between the key and the caliper, increases the frictional resistance, i.e. "effective sliding friction", noted above, and with galling or binding rust accumulation, the effective sliding friction between the caliper and key can rapidly become greater than the force provided by road vibration for moving engaged braking surfaces away from the disc.
As noted above, the support spring provides an effective filler or spacer between the caliper sliding surfaces and the key sliding surfaces. The intended clearance between the caliper sliding surfaces and the key sliding surface is generally in the range of 50-70 mils, i.e. only about a 20 mil variation in dimensional tolerance. However, due to manufacturing tolerances in producing disc brakes, that clearance can easily range between 40 and 100 mils. In order to compensate for this range of dimensional tolerance encountered in normal manufacture, the support spring must have the above noted relatively large g compression factor. With this higher g compression factor, the spring can accommodate variable manufacturing dimensional tolerances considerably greater than those intended. This distance is important from a safety point of view, since the spring is required to securely engage the caliper, in a slideable manner, with the key mounted on the anchor frame and to prevent substantial displacement of the caliper from the key in a generally vertical direction. If this distance between the caliper and the key is too great, it is possible for the caliper to rotate, in the vertical direction, off of the key especially in the event of spring failure and ensuing loss of spring, and cause considerable damage to the vehicle on which the brake is mounted, as well as be the cause of a serious accident. The relatively large g compression factor of the spring, thus, serves to insure that the caliper will not be rotated off of the key.
With heavy support springs, of the fore-going nature, i.e. large g loads for full compression, between the caliper sliding surface and the key sliding surface, the "effective sliding friction" between the caliper sliding surface and the key sliding surface is considerably increased. In some cases, where manufacturing tolerances result in close clearance between the caliper sliding surface and the key sliding surface, e.g. 40 mils or less, the disposition of the heavy support spring therebetween can increase the "effective sliding friction" to that which will prevent disengagement of the brake pads by normal road vibration, even in the essential absence of rust and contamination, e.g. on a new brake. This further accentuates the problem noted above. . .
The above noted problems, in general, have been recognized by the art, but the specific causes for those general problems have heretofore eluded the art. For example, one approach in the art to avoid the overall problem, i.e. binding of the sliding surfaces of the key and the caliper, is by protecting those sliding surfaces from intrusion of moisture and atmospheric contamination, including dirt, dust and the like. These protecting devices, are generally referred to as dust covers, although they function in manners other than covers, particularly in special brake designs. Representative of such approaches in various brake designs are U.S. Pat. Nos. 3,997,032; 4,084,666; and 4,162,721.
Another approach in the art is that of providing some degree of corrosion resistance to sliding parts in the disc brake, with or without additional protection such as dust covers. For example, U.S. Pat. No. 4,046,234 suggests placing a thin sheet of corrosive resistant metal, such as stainless steel, on sliding surfaces between a fixed portion of the brake (e.g. the frame) and the caliper body.
U.S. Pat. No. 4,189,032, recognizes the problem of rusting between the sliding surfaces, and further recognizes that the rusting problem can be mitigated by providing large clearances between those sliding surfaces. However, as that patent points out, such large clearances produce noisy brakes due to the "clunk" which occurs, for the reasons noted above. That patent suggests that the key and keyway be replaced by a pin within a bore, in order to protect the pin from the elements. To further protect the pin from the elements, that patent suggests a dust cover (or boot), as well as providing a non-corrosive bushing, e.g. brass, in the bore and a non-corrosive pin, e.g. stainless steel. However, this approach is considerably more expensive than the conventional disc brake design, described above, particularly in that the bore, bushing and pin must be very accurately manufactured. In addition, since the clearance between the bushing and pin must be quite small, for operation of the pin type brake, any failure of the dust cover which would allow contaminates to enter between the bushing and pin can cause considerable difficulty in operation of the brake. Further, in this arrangement, since the caliper will not "float" and with a key and keyway arrangement, it has no ability to compensate automatically for slight warping of the disc or for any lack of parallelism between the axis of the disc and the direction of the caliper sliding along the keyway.
In order to retain the advantage of the "floating" caliper disc brake, for the reasons noted in the foregoing paragraph, another approach to the problem is that of providing specially configured keys. Thus, U.S. Pat. No. 4,109,764, suggests a generally V-shaped cross-section key with a cylindrical surface on one arm of the V. This arrangement is said to permit slight pivoting of the caliper relative to the fixed member, e.g. a type of "floating" action, while discouraging penetration by impurities to the sliding surfaces. A variety of other special key designs are known to the art, but these key designs, in general, restrict the "floating" movement of the caliper on the anchor frame and in a more limited regard suffer from the same disadvantages of the pin approach, discussed above. Further, in general, these special key designs require more intricate machining of the parts of the caliper and anchor frame, as well as more complicated and expensive key designs.
Another approach, somewhat similar to the foregoing, is that of providing special torque members, such as disclosed in U.S. Pat. No. 4,134,477, but here again, this approach considerably increases the cost of the disc brake.
Accordingly, the problem described above has never been adequately solved, in that prior art approaches have either considerably increased the cost of the brake or have resulted in restriction of the "floating" action, which "floating" action is quite considerable in disc brakes. It would, therefore, be of considerable advantage in the art to provide a solution to this problem, which neither substantially increases the cost of the brake nor significantly reduces the "floating" action of the brake. It would be a further advantage to the art to provide this solution such that it can be applied to the original manufacture of brakes or to the repair of existing brakes. This latter consideration is of considerable importance, bearing in mind the very large number of brakes now in service which suffer from that problem.